Acetic acid is a basic chemical product having a wide range of uses such as raw material for poly vinyl acetate, acetylcellulose and ester acetates, solvent in manufacturing plant for terephthalic acid, and the like.
Known methods for producing acetic acid used in chemical industry include carbonylation of methanol, partial oxidation of acetaldehyde, and oxidation of butane or propane. Presently, carbonylation of methanol is performed for the most part thereof.
As a method for producing acetic acid by carbonylation of methanol, “Monsanto process” (Patent Literature 1) using the reaction of methanol and carbon monoxide in acetic solvent containing water, with rhodium compound and methyl iodide homogeneously dissolved therein, is well known.
Although this method has an advantage in producing acetic acid from methanol with a high yield, there still have existed the following problems associated with the method.
First, coexistence of highly-concentrated water is required to keep a high carbonylation reaction rate and high acetic acid selectivity. The highly-concentrated water accelerates the hydrolysis of methyl iodide which is used as a co-catalyst in this method, and produces a large quantity of hydroiodic acid. The hydroiodic acid corrodes the equipment system. Furthermore, due to the necessity of separation of the water and hydroiodic acid mixed in acetic acid, it increases the load of the refining process for the acetic acid production, which results in high production cost. Second, due to the low solubility of the catalytic metal complex in solvent, a high reaction rate cannot be obtained. And third, the dissolved catalyst precipitates in the steps of separation and refinement, which substantially increases the cost and load for the separation and recovery thereof.
In order to solve these problems, an advanced method that the reaction proceeded in a heterogeneous system having rhodium supported on a pyridine resin carrier (Patent Literatures 2 to 5) has been proposed and put into practical use.
According to these prior arts, firstly, a high carbonylation reaction rate and a high acetic acid selectivity can be kept even under such conditions where reaction system has a lower water concentration of not higher than 5 wt %. Since producing of acetic acid can be done under a lower water concentration condition, the amount of hydroiodic acid produced by the hydrolysis of methyl iodide as a co-catalyst is reduced, resulting in reduced risk of corroding of materials used in the equipment system. In addition, the easiness of separation, recovery, and refinement of the produced acetic acid is enabled, which leads to reduced load in the refining process for the acetic acid production, resulting in an advantage of a lower cost. Secondly, since the rhodium complex is supported, a high concentration of catalyst can be achieved despite the low solubility of the rhodium complex, resulting in an enhanced reaction rate. And thirdly, since the catalyst is contained in the reactor, loss of rhodium can be reduced in the steps of separation and refinement.
According to the Patent Literature 3, preferably the carbonylation reaction of methanol is performed in the region where liquid reaction product has a carbonylation degree Ca of not lower than 0.8 mol/mol.
A carbonylation degree Ca is a value defined by the following expression.Ca=Σin Ci·Zi/Σin Ci·Xi  [Expresssion1]
In the expression, Ci represents a molar concentration (mol/l) of each component Mi existing in solution, Zi represents a carbonylation factor of each component Mi, and Xi represents a raw material factor of each component Mi, respectively. And n represents the total number of the components Mi existing in solution. The carbonylation factor Zi and the raw material factor Xi of each component Mi are shown in the following table.
TABLE 1CarbonylationRaw materialComponent (Mi)factor (Zi)factor (Xi)Methyl iodide00Methanol01Acetic acid11Methyl acetate12Water00Dimethyl ether02Carbon monoxide00Other organic compound00
In the carbonylation reaction of methanol, the main reaction represented by the following reaction formula (1) involves side reactions represented by the following reaction formulae (2) and (3).CH3OH+COCH3COOH  (1)CH3COOH+CH3OHCH3COOCH3+H2O  (2)2CH3OHCH3OCH3+H2O  (3)CH3I+H2OCH3OH+HI  (4)
The by-product water produced from the side reactions (2) and (3) produces hydroiodic acid by hydrolysis, as shown in (4), of methyl iodide as a co-catalyst, which may corrode the equipment system. And due to the necessity of separation of the by-product water and hydroiodic acid mixed in acetic acid, the load in the refining process for acetic acid production increases, which may further result in a high production cost.
In contrast, performing of reaction in the region at a high carbonylation degree can inhibit the side reactions (2) and (3), and thus reduces the production of hydrogen iodide in (4). In addition, by inhibiting the production of water and methyl acetate as by-products, acetic acid concentration in reaction products can be increased, resulting in an advantage of lower cost for refinement.